The present invention relates to the field of antennas and, more particularly, to an antenna feed system which corrects and improves the performance of antenna feeds used in parabolic antennas.
In the simplest configuration, an antenna feed is located at the focal point of a parabolic dish antenna. The feed usually consists of a scalar plate, a circular or square waveguide input and a orthomode coupler of some kind. The orthomode coupler separates orthogonal electromagnetic fields such as vertical and horizontally polarized waves. When only one polarization is used, the waveguide input may be rectangular. The diameter of the circular or square waveguide input effectively controls the feed primary pattern, such as beam width and frequency response. The scalar plate helps the feed primary pattern E and H-plane cuts to coincide.
Once the frequency of the antenna is known, the diameter of the waveguide input is chosen to allow the electromagnetic wave to propagate. Hollow pipes, such waveguide, have a cut-off frequency for which propagation is allowed above a certain frequency known as the cut-off frequency. The cut-off frequency is directly related to the pipe circular diameter. For square or rectangular waveguide, the cut-off frequency is related to the side dimensions. As the diameter increases, the cut-off frequency decreases. At the same time, the feed primary pattern beam width is controlled by this same diameter. As the diameter increases, the primary pattern beam width decreases.
For parabolic antennas, the antenna designer uses the feed to illuminate the dish. The feed is located at the parabolic dish focal point and aimed at the dish center as shown in FIG. 1. In receiving applications, electromagnetic waves striking the dish are focused onto the feed. When transmitting, the power from the feed illuminates the dish. The feed and antenna behave the same independent of whether the dish and feed are used for transmitting or receiving. The feed designer tries to illuminate the dish in such a way as to make the primary pattern roll-off about 10 dB at the antenna edge. For satellite communications applications, the roll-off insures that the ground in not illuminated by an appreciable amount of the primary pattern power that spills off the parabolic dish edge. With a smaller roll-off, to much of the ground is illuminated making the antenna receive system noisy. With a larger roll-off, the dish is under illuminated making the dish gain low.
To help with the understanding of the feed design problem, let us first explore the antenna geometry schematically shown in FIG. 2. A parabolic dish antenna is defined by its focal length and diameter. Engineers define the parabolic geometry by the antenna f/d ratio where f is the focal distance and d is the dish diameter. The dish edge makes an angle A(x) with respect to the perpendicular from the dish center to focal point given by the following formula, where x is the f/d ratio. For small, f/d ratios usually encountered in satellite dish antennas, the edge angle is quite large.       A    ⁡          (      x      )        =      atan    ⁢          xe2x80x83        ⁢          "AutoLeftMatch"              (                  0.5                      x            -                          1                              16                ⁢                x                                                    )            
For f/d=0.3, A(0.3)=79.6 degrees.
For the above example, the feed designer""s problem is roll-off the feed primary pattern approximately 10-dB at dish edge 79.6 degrees from the center of the dish. The problem the feed designer runs into however is related to the diameter of the feed input waveguide. With any choice of the feed input waveguide diameter above the cut-off frequency, the dish is usually not illuminated properly. Almost any choice of waveguide diameter makes the roll-off to largexe2x80x94on the order 18 to 25 dB. To broaden the primary feed pattern and reduce the roll-off, the feed designer then tries to do is to make the input waveguide diameter smaller. Before anything meaningful is achieved, the designer runs into the cut-off frequency. In summary, the problem is that the feed designer runs into the cut-off frequency before he can improve the roll-off.